This invention relates to spacing devices for spacing the individual conductors of one phase or pole of an overhead electrical power transmission line, the device also serving to damp the relative movements of the conductors. For this reason, the devices are often referred to as spacer-dampers.
Our British Pat. No. 966,243, corresponding to U.S. Pat. No. 3,157,731, discloses a spacer-damper that has proved very effective and is in widespread use. It consists essentially of a hub having at least two cavities each opening onto the hub periphery, and at least two arms each having a conductor clamp at one end and a knob at the other end, the knob being received with clearance within a respective cavity to permit limited articulation of each arm with respect to the hub. An elastomeric bush encircles each arm in the gap between the knob and the cavity wall so that oscillations of the conductors are damped by straining the elastomeric material, the kinetic energy being dissipated as heat. In general, the greater the amplitude of the conductor oscillations, the greater will be the deflection of the elastomer and the greater will be the energy dissipated per cycle. Thus to accommodate as large an amplitude of conductor movement as possible (without the conductors actually touching one another), the bush should generally be as fat as possible, consistent with a good fatigue life. Also in general the lower the stiffness of the spacer, the larger will be the permitted amplitude of conductor movement.
However, merely fattening the bush disclosed in our aforesaid U.S. Pat. No. 966,243 in order to accommodate bigger deflections of the elastomeric material will lead to a reduction in fatigue life of the spacer. This is because there will be an increased chance of slippage occurring at the contact face between the knob and the bush when the arm is displaced from its neutral position. Because the bush is compressed between the knob and the cavity wall during assembly of the spacer, any slippage as a result of large conductor movements, will cause wear and chafing of the elastomer. This could eventually lend to loss of the pre-compression and thereby reduce the effectiveness of the bush. The pre-compression is intended to prevent slippage but, since the elastomer is generally a constant volume material, any attempt to increase this pre-compression for a given clearance gap by increasing the fatness of the bush before compression will only result in the excess elastomer being squeezed out of the gap.
In accordance with the present invention this problem is overcome by shaping the cross-section of each half of the uncompressed bush located on opposing sides of the maximum diameter portion of the knob such that the cross-section of each half bush is deformed during compression to displace elastomeric material laterally in both directions away from the centre of each half bush whereby the compressed halves of the bush each have a greater degree of pre-compression at the centre of their cross-section than at the ends of their cross-section. Each half bush may be split into two semi-circular bushes which together encircle the arm, and the two pairs of half bushes on opposite sides of the maximum diameter portion of the knob are then preferably interconnected, for example by a small integral elastomeric tab. This facilitates assembly of the spacer since it avoids having four separate half bushes.
Each bush is compressed in the gap between the knob and the cavity wall during assembly of the device. Before compression each half bush preferably has a cross-section which over its centre portion is considerably thicker than the pertinent gap dimension and which over its two side portions is thinner than the pertinent gap dimension. Thus, when the half bush is compressed in the relevant gap, the elastomeric material is displaced both ways from the centre portion and accommodated at the side portions. In a preferred form the gap is a parallel-sided gap in cross-section and each uncompressed half bush is generally semi-circular in cross-section with the exposed diameter abutting one side of the gap. When compressed, the generally semi-circular cross-section shape deforms to a generally rectangular shape with rounded corners.